Apparatus for and method for polishing workpiece

ABSTRACT

A polishing apparatus for polishing a workpiece such as a semiconductor wafer has a turntable with a polishing surface, and a top ring for holding a workpiece and pressing the workpiece against the polishing surface under a first pressing. The polishing apparatus has a pressurized fluid source for supplying pressurized fluid, and a plurality of openings provided in the holding surface of the top ring for ejecting the pressurized fluid supplied from the pressurized fluid source. A plurality of areas each having the openings are defined on the holding surface so that the pressurized fluid is selectively ejectable from the openings in the respective areas.

REFERENCE TO RELATED APPLICATION

A continuation of the reissue application was filed May 13, 2002 andbears Ser. No. 10/142,980.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus for and a method ofpolishing a workpiece such as a semiconductor wafer to a flat mirrorfinish, and more particularly to an apparatus for and a method ofpolishing a workpiece such as a semiconductor wafer which can controlthe amount of a material removed from a desired area of the workpiece bya polishing action.

2. Description of the Related Art

Recent rapid progress in semiconductor device integration demandssmaller and smaller wiring patterns or interconnections and alsonarrower spaces between interconnections which connect active areas. Oneof the processes available for forming such interconnection isphotolithography. Though the photolithographic process can forminterconnections that are at most 0.5 μm wide, it requires that surfaceson which pattern images are to be focused by a stepper be as flat aspossible because the depth of focus of the optical system is relativelysmall.

It is therefore necessary to make the surfaces of semiconductor wafersflat for photolithography. One customary way of flattening the surfacesof semiconductor wafers is to polish them with a polishing apparatus.

Conventionally, a polishing apparatus has a turntable and a top ringwhich rotate at respective individual speeds. A polishing cloth isattached to the upper surface of the turntable. A semiconductor wafer tobe polished is placed on the polishing cloth and clamped between the topring and the turntable. An abrasive liquid containing abrasive grains issupplied onto the polishing cloth and retained on the polishing cloth.During operation, the top ring exerts a certain pressure on theturntable, and the surface of the semiconductor wafer held against thepolishing cloth is therefore polished to a flat mirror finish while thetop ring and the turntable are rotating.

The polishing apparatus is required to have such performance that thesurfaces of semiconductor wafers have a highly accurate flatness.Therefore, it is preferable that the lower end surface (the holdingsurface) of the top ring which holds a semiconductor wafer and thecontact surface of the polishing cloth which is held in contact with thesemiconductor wafer, and hence the surface of the turntable to which thepolishing cloth is attached, have a highly accurate flatness, and thosesurfaces which are highly accurately flat have been used in the art. Thelower surface of the top ring and the upper surface of the polishingcloth are parallel to each other as in the ordinal cases.

It is known that the polishing action of the polishing apparatus isaffected not only by the configurations of the holding surface of thetop ring and the contract surface of the polishing cloth, but also bythe relative speed between the polishing cloth and the semiconductorwafer, the distribution of pressure applied to the surface of thesemiconductor wafer which is being polished, the amount of the abrasiveliquid on the polishing cloth, and the period of time when the polishingcloth has been used. It is considered that the surface of thesemiconductor wafer can be highly accurately flat if the above factorswhich affect the polishing action of the polishing apparatus areequalized over the entire surface of the semiconductor wafer to bepolished. The larger the size of the semiconductor wafer is, the moredifficult the above factors are equalized.

However, some of the above factors can easily be equalized over theentire surface of the semiconductor wafer, but the other factors cannotbe equalized. For example, the relative speed between the polishingcloth and the semiconductor wafer can easily be equalized by rotatingthe turntable and the top ring at the same rotational speed and in thesame direction. However, it is difficult to equalize the amount of theabrasive liquid on the polishing cloth because of a centrifugal forcesimposed on the abrasive liquid.

The above approach which tries to equalize all the factors affecting thepolishing action, including the flatnesses of the lower end surface ofthe top ring and the upper surface of the polishing cloth on theturntable, over the entire surface of the semiconductor wafer to bepolished poses limitations on efforts to make the polished surface ofthe semiconductor wafer flat, often resulting in a failure to accomplisha desired degree of flatness of the polished surface.

It has been customary to achieve a more accurate flatness by making theholding surface of the top ring concave or convex to develop a certaindistribution of pressure on the surface of the semiconductor wafer forthereby correcting irregularities of the polishing action which arecaused by an irregular entry of the abrasive liquid and variations inthe period of time when the polishing cloth has been used.

However, various problems have arisen in the case where a specificconfiguration is applied to the holding surface of the top ring.Specifically, since the holding surface of the top ring is held incontact with the semiconductor wafer at all times, the holding surfaceof the top ring affects the polishing action continuously all the timewhile the semiconductor wafer is being polished. Because theconfiguration of the holding surface of the top ring has direct effecton the polishing action, it is highly complex to correct irregularitiesof the polishing action by intentionally making the holding surface ofthe top ring concave or convex, i.e., non-flat. If the holding surfaceof the top ring which has been made intentionally concave or convex isinadequate, the polished surface of the semiconductor wafer may not bemade as flat as desired, or irregularities of the polishing action maynot be sufficiently corrected, so that the polished surface of thesemiconductor wafer may not be sufficiently flat.

In addition, inasmuch as the holding surface of the top ring is ofsubstantially the same size as the surface of the semiconductor wafer tobe polished, the holding surface of the top ring is required to be madeirregular in a very small area. Because such surface processing ishighly complex, it is not easy to correct irregularities of thepolishing action by means of the configuration of the holding surface ofthe top ring.

The conventional polishing apparatuses, particularly those for polishingsemiconductor wafers, are required to polish workpiece surfaces tohigher flatness. There have not been available suitable means andapparatus for polishing workpieces to shapes which are intentionally notflat or for polishing workpieces such that desired localized areas ofworkpiece surfaces are polished to different degrees.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide apolishing apparatus which can easily correct irregularities of apolishing action on a workpieces such as a semiconductor wafer, andpolish a workpiece with an intensive polishing action on a desiredlocalized area thereof.

According to an aspect of the present invention, there is provided apolishing apparatus for polishing a surface of a workpiece comprising: aturntable having a polishing surface thereon; a top ring for supportingthe workpiece to be polished and pressing the workpiece against thepolishing surface under a first pressing force, the top ring having aholding surface for holding the workpiece; a pressurized fluid sourcefor supplying pressurized fluid; a plurality of openings provided in theholding surface of the top ring for ejecting the pressurized fluidsupplied from the pressurized fluid source, a plurality of areas eachhaving the openings being defined in the holding surface so that thepressurized fluid is selectively ejectable from the openings in therespective areas.

According to another aspect of the present invention, there is provideda method of polishing a workpiece, comprising the steps of: holding aworkpiece between a polishing surface of a turntable and a holdingsurface of a top ring disposed above the turntable; pressing theworkpiece by the top ring against the polishing surface under a firstpressing force; and ejecting pressurized fluid from openings in aplurality of areas in the holding surface of the top ring toward theworkpiece held by the top ring, the pressurized fluid being selectivelyejectable from the openings in the respective areas; and polishing theworkpiece in such a state that a pressing force applied to the workpieceby the pressurized fluid is variable in a central portion and an outercircumferential portion of the workpiece, respectively.

The above and other objects, features, and advantages of the presentinvention will become apparent from the following description when takenin conjunction with the accompanying drawings which illustrate preferredembodiments of the present invention by way of example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary vertical cross-sectional view showing the basicprinciples of the present invention;

FIGS. 2A, 2B, and 2C are enlarged fragmentary vertical cross-sectionalviews showing the behavior of an polishing cloth when the relationshipbetween a pressing force applied by a top ring and a pressing forceapplied by a presser ring is varied;

FIGS. 3A through 3C are graphs showing the results of an experiment inwhich a semiconductor wafer was polished based on the basic principlesof the present invention;

FIGS. 4A through 4E are graphs showing the results of an experiment inwhich a semiconductor wafer was polished based on the basic principlesof the present invention;

FIG. 5 is a vertical cross-sectional view of a polishing apparatusaccording to a first embodiment of the present invention;

FIG. 6 is an enlarged vertical cross-sectional view showing details of atop ring and a presser ring of the polishing apparatus according to thefirst embodiment;

FIG. 7 is a cross-sectional view taken along line VII—VII of FIG. 6; and

FIG. 8 is an enlarged vertical cross-sectional view of a polishingapparatus according to a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Like or corresponding parts are denoted by like or correspondingreference numerals throughout views.

FIG. 1 shows the basic principles of the present invention. As shown inFIG. 1, a top ring 1 has therein a circular first chamber C₁ at acentral position thereof, an annular second chamber C₂ disposed at aradially outer side of the first chamber C₁, and an annular thirdchamber C₃ disposed at a radially outer side of the second chamber C₂.The first chamber C₁ is connected to a pressurized fluid source througha valve V₁, the second chamber C₂ is connected to a pressurized fluidsource through a valve V₂, and the third chamber C₃ is connected to apressurized fluid source through a valve V₃. The top ring 1 has a recess1a defined in a lower surface thereof for accommodating therein asemiconductor wafer 4 which is a workpiece to be polished. An elasticpad 2 of polyurethane or the like is attached to the lower surface ofthe top ring 1.

The top ring 1 and the elastic pad 2 have a plurality of openings 1o and2o, respectively, which are in registry with each other. Each of theopenings 1o and 2o is communicated with anyone of the first chamber C₁,the second chamber C₂, and the third chamber C₃. That is, a plurality ofopenings each comprising the openings 1o and 2o for ejecting pressurizedfluid are provided in a holding surface of the top ring 1 for holdingthe semiconductor wafer 4 to be polished. Thus, three concentric annularareas are defined on the holding surface of the top ring 1 by allowingthe openings 1o and 2o to be communicated with anyone of the first,second and third chambers C₁, C₂ and C₃. The pressurized fluid isejectable from the openings in the respective annular areas, separately.

A presser ring 3 is disposed around the top ring 1 and is verticallymovable with respect to the top ring 1. A turntable 5 having an uppersurface to which a polishing cloth 6 is attached is provided below thetop ring 1. The top ring 1 applies a pressing force F₁ (pressure perunit area, gf/cm²) to press the semiconductor wafer 4 against thepolishing cloth 6 on the turntable 5, and the presser ring 3 applies apressing force F₂ (pressure per unit area, gf/cm²) to press thepolishing cloth 6. These pressing forces F₁, F₂ are variableindependently of each other.

During polishing, pressurized fluid such as compressed air is suppliedto the first, second and third chambers C₁, C₂ and C₃, selectively, andthe supplied pressurized fluid is ejected from the lower surface of theelastic pad 2 through the openings 1o and 2o and is supplied between theholding surface of the top ring 1 and the upper surface of thesemiconductor wafer 4. At this time, at least one of the first, secondand third chambers C₁, C₂ and C₃ to which pressurized fluid is suppliedis selected, and hence at least one of the annular areas, from whichpressurized fluid is ejected, in the holding surface of the top ring 1is selected. For example, pressurized fluid is supplied only to thefirst chamber C₁, and is not supplied to the second and third chambersC₂ and C₃, and thus the pressurized fluid is ejected only from thecentral area of the holding surface of the top ring 1. As a result, thesemiconductor wafer 4 is pressed against the polishing cloth 6 by thepressurized fluid in such a state that the polishing pressure applied tothe central portion of the semiconductor wafer 4 is larger than thepolishing pressure applied to outer circumferential portion of thesemiconductor wafer 4. Thus, if the amount of a material removed fromthe outer circumferential portion of the semiconductor wafer 4 is largerthan the amount of a material removed from the central portion of thesemiconductor wafer 4, insufficient polishing action at the centralportion of the semiconductor wafer can be corrected by utilizing thepressing action of the pressurized fluid.

On the other hand, if the amount of a material removed from the centralportion of the semiconductor wafer 4 is larger than the amount of amaterial removed from the outer circumferential portion of thesemiconductor waiter 4, the pressurized fluid is supplied only to thethird chamber C₃, and is not supplied to the first and second chambersC₁ and C₂, and thus the pressurized fluid is ejected only from the outercircumferential area of the holding surface of the top ring 1.

As a result, the polishing pressure applied to the outer circumferentialportion of the semiconductor wafer 4 is made larger than the centralportion of the semiconductor wafer 4. Thus, insufficient polishingaction at the outer circumferential portion of the semiconductor wafercan be collected, and the entire surface of the semiconductor wafer 4can be uniformly polished.

The pressures of pressurized fluid supplied to the first chamber C₁, thesecond chamber C₂ and the third chamber C₃ are changed, respectively.That is, pressurized fluid having a pressure of p₁ gf/cm² is supplied tothe first chamber C₁, pressurized fluid having a pressure of P₂ gf/cm²is supplied to the second chamber C₂, and pressurized fluid having apressure of p₃ gf/cm² is supplied to the third chamber C₃, respectively.In this manner, the pressures of pressurized fluid ejected from therespective annular areas of the holding surface of the top ring 1 arevaried, and the fluid which is supplied between the holding surface ofthe top ring 1 and the upper surface of the semiconductor wafer 4 haspressure gradient so as to be higher or lower progressively from thecentral area to the outer circumferential area of the semiconductorwafer 4, and hence the pressing force for pressing the semiconductorwafer 4 against the polishing cloth 6 has gradient so as to be higher orlower progressively from the central area to the outer circumferentialarea of the semiconductor wafer 4. Thus, irregularities of the polishingaction can be sufficiently corrected, and the localized area of thesemiconductor wafer 4 is prevented from being polished excessively orinsufficiently.

In the present invention, the pressing force F₁ (pressure per unit area,gf/cm²) for pressing the semiconductor wafer 4 against the polishingcloth 6, and the pressing force F₂ (pressure per unit area, gf/cm²) forpressing the polishing cloth 6 are variable independently of each other.Therefore, the pressing force F₂ which is applied to the polishing cloth6 by the presser ring 3 can be changed depending on the pressing forceF₁ which is applied by the top ring 1 to press the semiconductor wafer 4against the polishing cloth 6.

Theoretically, if the pressing force F₁ which is applied by the top ring1 to press the semiconductor wafer 4 against the polishing cloth 6 isequal to the pressing force F₂ which is applied to the polishing cloth 6by the presser ring 3, then the distribution of applied polishingpressures, which result from a combination of the pressing forces F₁,F₂, is continuous and uniform from the center of the semiconductor wafer4 to its peripheral edge and further to an outer circumferential edge ofthe presser ring 3 disposed around the semiconductor wafer 4.Accordingly, the peripheral portion of the semiconductor wafer 4 isprevented from being polished excessively or insufficiently.

FIGS. 2A through 2C schematically show how the polishing cloth 6 behaveswhen the relationship between the pressing force F₁ and the pressingforce F₂ is varied. In FIG. 2A, the pressing force F₁ is larger than thepressing force F₂ (F₁>F₂). In FIG. 2B, the pressing force F₁ is nearlyequal to the pressing force F₂ (F₁≈F₂). In FIG. 2C, the pressing forceF₁ is smaller than the pressing force F₂ (F₁<F₂).

As shown in FIGS. 2A through 2C, when the pressing force F₂ applied tothe polishing cloth 6 by the presser ring 3 is progressively increased,the polishing cloth 6 pressed by the presser ring 3 is progressivelycompressed, thus progressively changing its state of contact with theperipheral portion of the semiconductor wafer 4, i.e., progressivelyreducing its area of contact with the peripheral portion of thesemiconductor wafer 4. Therefore, when the relationship between thepressing force F₁ and the pressing force F₂ is changed in variouspatterns, the distribution of polishing pressures on the semiconductorwafer 4 over its peripheral portion and inner region is also changed invarious patterns.

As shown in FIG. 2A, when the pressing force F₁ is larger than thepressing force F₂ (F₁>F₂), the polishing pressure applied to theperipheral portion of the semiconductor wafer 4 is larger than thepolishing pressure applied to the inner region of the semiconductorwafer 47 so that the amount of a material removed from the peripheralportion of the semiconductor wafer 4 is larger than the amount of amaterial removed from the inner region of the semiconductor wafer 4while the semiconductor wafer 4 is being polished.

As shown in FIG. 2B, when the pressing force F₁ is substantially equalto the pressing force F₂ (F₁≈F₂), the distribution of polishingpressures is continuous and uniform from the center of the semiconductorwafer 4 to its peripheral edge and further to the outer circumferentialedge of the presser ring 3, so that the amount of a material removedfrom the semiconductor wafer 4 is uniform from the peripheral edge tothe inner region of the semiconductor wafer 4 while the semiconductorwafer 4 is being polished.

As shown in FIG. 2C, when the pressing force F₁ is smaller than thepressing force F₂ (F₁<F₂), the polishing pressure applied to theperipheral portion of the semiconductor wafer 4 is smaller than thepolishing pressure applied to the inner region of the semiconductorwafer 4, so that the amount of a material removed from the peripheraledge of the semiconductor wafer 4 is smaller than the amount of amaterial removed from the inner region of the semiconductor wafer 4while the semiconductor wafer 4 is being polished.

The pressing force F, and the pressing force F₂ can be changedindependently of each other before polishing or during polishing.

As described above, according to the present invention, pressurizedfluid is ejected from the holding surface of the top ring 1. At thistime, the areas from which the pressurized fluid is ejected are suitablyselected, and the pressing force applied to the semiconductor wafer 4 bythe pressurized fluid is changed in the central portion and the outercircumferential portion of the semiconductor wafer 4, respectively,during polishing.

In parallel with the above process, the pressing force F₂ of the presserring 3 disposed around the top ring 1 is determined on the basis of thepressing force F₁ of the top ring 1, and the semiconductor wafer 4 ispolished while pressing the polishing cloth 6 by the presser ring 3under the pressing force F₂ which has been determined. Further, thepressing force F₂ is determined on the basis of the pressuredistribution which is applied to the semiconductor wafer 4 by thepressurized fluid, and the semiconductor wafer 4 is polished by acombination of an action caused by the pressurized fluid and an actioncaused by the presser ring 3. In this manner, insufficient polishingaction in thus localized area (for example, the central area or theouter circumferential area) of the semiconductor wafer can be corrected,and the localized area of the semiconductor wafer is prevented frombeing polished excessively or insufficiently. In the case where thepolishing pressure applied to the central portion of the semiconductorwafer 4 is made larger than the outer circumferential portion of thesemiconductor wafer 4 by supplying the pressurized fluid, the pressingforce F₂ of the presser ring 3 is made larger than the pressing force F₁of the top ring 1. Conversely, in the case where the polishing pressureapplied to the outer circumferential portion of the semiconductor wafer4 is made larger than the central portion of the semiconductor wafer 4by supplying the pressurized fluid, the pressing force F₂ of the presserring 3 is made smaller than the pressing force F₁ of the top ring 1.

FIGS. 3A through 3C show the results of an experiment in which asemiconductor wafer was polished based on the basic principles of supplyof pressurized fluid according to the present invention. Thesemiconductor wafer used in the experiment was an 8-inch semiconductorwafer. In the experiment, the pressing force (polishing pressure)applied to the semiconductor wafer by the top ring was a constant levelof 400 gf/cm², and the supply of the pressurized fluid was controlled.FIG. 3A shows the case in which the pressurized fluid was not supplied.FIG. 3B shows the case in which the pressurized fluid is supplied onlyto the first chamber C₁, and FIG. 3C shows the case in which thepressurized fluid is supplied only to the third chamber C₃. The pressureof the pressurized fluid was 200 gf/cm². In each of FIGS. 3A through 3C,the horizontal axis represents a distance(mm) from the center of thesemiconductor wafer, and the vertical axis represents a thickness (Å) ofa material removed from a semiconductor wafer.

As shown in FIGS. 3A through 3C, the thickness of the removed materialat the radial positions on the semiconductor wafer is affected bycontrolling the supply of the pressurized fluid. Specifically, when thepressurized fluid was not supplied, as shown in FIG. 3A, the peripheralportion of the semiconductor wafer was excessively polished. When thepressurized fluid is supplied only to the first chamber C₁ to press onlythe central portion of the semiconductor wafer by the pressurized fluid,as shown in FIG. 3B, the peripheral portion of the semiconductor waferwas not excessively polished and the central portion of thesemiconductor wafer was slightly excessively polished. When thepressurized fluid was supplied only to the third chamber C₃ to pressonly the outer circumferential portion of the semiconductor wafer by thepressurized fluid, as shown in FIG. 3C, the outer circumferentialportion of the semiconductor wafer was excessively polished and thecentral portion of the semiconductor wafer was polished insufficiently.

As described above, the experimental result shown in FIGS. 3A through 3Eindicate that the amount of the material removed from the localized areaof the semiconductor wafer can be adjusted by controlling supply of thepressurized fluid.

FIGS. 4A through 4E show the results of an experiment in which asemiconductor wafer was polished based on the basic principles of thepresent invention. The semiconductor wafer used in the experiment was an8-inch semiconductor wafer. In the experiment, the pressing force(polishing pressure) applied to the semiconductor wafer by the top ringwas a constant level of 400 gf/cm², and the pressing force applied bythe presser ring was changed from 600 to 200 gf/cm² successively bydecrements of 100 gf/cm². Specifically, the pressing force applied bythe presser ring was 600 gf/cm² in FIG. 4A, 500 gf/cm² in FIG. 4B, 400gf/cm² in FIG. 4C, 300 gf/cm² in FIG. 4D, and 200 gf/cm² in FIG. 4E. Ineach of FIGS. 4A through 4E, the horizontal axis represents a distance(mm) from the center of the semiconductor wafer, and the vertical axisrepresents a thickness (Å) of a material removed from the semiconductorwafer.

As shown in FIGS. 4A through 4E, the thickness of the removed materialat the radial positions on the semiconductor wafer is affected when thepressing force applied by the presser ring was changed. Specifically,when the pressing force applied by the presser ring was in the rangefrom 200 to 300 gf/cm² as shown in FIGS. 4D and 4E, the peripheralportion of the semiconductor wafer was excessively polished. When thepressing force applied by the presser ring was in the range from 400 to500 gf/cm², as shown in FIGS. 4B and 4C, the peripheral portion of thesemiconductor wafer is substantially equally polished from theperipheral edge to the inner region of the semiconductor wafer. When thepressing force applied by the presser ring was 600 gf/cm² ₁ as shown inFIG. 4A, the peripheral portion of the semiconductor wafer was polishedinsufficiently.

The experimental results shown in FIGS. 4A through 4E indicate that theamount of the material removed from the peripheral portion of thesemiconductor wafer can be adjusted by varying the pressing forceapplied by the presser ring independently of the pressing force appliedby the top ring. From a theoretical standpoint, the peripheral portionof the semiconductor wafer should be polished optimally when thepressing force applied by the presser ring is equal to the pressingforce applied by the top ring. However, since the polishing actiondepends on the type of the semiconductor wafer and the polishingconditions, the pressing force applied by the presser ring is selectedto be of an optimum value based on the pressing force applied by the topring depending on the type of the semiconductor wafer and the polishingconditions.

There are demands for the removal of a larger or smaller thickness ofmaterial from the peripheral portion of the semiconductor wafer thanfrom the inner region of the semiconductor wafer depending on the typeof the semiconductor wafer. To meet such demands, the pressing forceapplied by the presser ring is selected to be of an optimum value basedon the pressing force applied by the top ring to intentionally increaseor reduce the amount of the material removed from peripheral portion ofthe semiconductor wafer.

FIGS. 5 through 7 show a polishing apparatus according to a firstembodiment of the present invention.

As shown in FIGS. 5 and 6, a top ring 1 has therein a circular firstchamber C₁ at a central position thereof, an annular second chamber C₂disposed at a radially outer side of the first chamber C₁, and anannular third chamber C₃ disposed at a radially outer side of the firstchamber C₂. The first chamber C₁ is connected to a compressed air source24 as a pressurized fluid source through a valve V₁ and a regulator R₁,the second chamber C₂ is connected to the compressed air source 24through a valve V₂ and a regulator R₂, and the third chamber C₃ isconnected to the compressed air source 24 through a valve V₃ and aregulator R₃. The top ring 1 has a recess 1a defined in a lower surfacethereof for accommodating therein a semiconductor wafer 4 which is aworkpiece to be polished. An elastic pad 2 of polyurethane or the likeis attached to the lower surface of the top ring 1.

The top ring 1 and the elastic pad 2 have a plurality of openings 1o and2o, respectively, which are in registry with each other. Each of theopenings 1o and 2o is communicated with anyone of the first chamber C₁,the second chamber C₂, and the third chamber C₃. That is, a plurality ofopenings each comprising the openings 1o and 2o for ejecting pressurizedfluid are defined on a holding surface of the top ring 1 for holding thesemiconductor wafer 4 to be polished. Thus, three concentric annularareas A₁, A₂ and A₃ are defined in the holding surface of the top ring 1by allowing the openings 1o and 2o to be communicated with anyone of thefirst, second and third chambers C₁, C₂ and C₃. The compressed airhaving different pressure from one another can be supplied to respectiveannular areas A₁, A₂ and A₃. Pressure gages or pressure sensors G₁, G₂and G₃ are provided in the respective pressurized fluid supply lines,and the pressure in the respective chambers C₁, C₂ and C₃ can beindependently controlled on the basis of the pressures detected by thepressure gages G₁, G₂ and G₃.

A presser ring 3 is disposed around the top ring 1 and is verticallymovable with respect to the top ring 1. A turntable 5 with a polishingcloth 6 attached to an upper surface thereof is disposed below the topring 1.

The top ring 1 is connected to a vertical top ring shaft 8 whose lowerend is held against a ball 7 mounted on an upper surface of the top ring1. The top ring shaft 8 is operatively coupled to a top ring aircylinder 10 fixedly mounted on an upper surface of a top ring head 9.The top ring shaft 8 is vertically movable by the top ring air cylinder10 to press the semiconductor wafer 4 supported on the elastic pad 2against the polishing cloth 6 on the turntable 5.

The top ring shaft 8 has an intermediate portion extending through andcorotatably coupled to a rotatable cylinder 11 by a key (not shown), andthe rotatable cylinder 11 has a pulley 12 mounted on outercircumferential surface thereof. The pulley 12 is operatively connectedby a timing belt 13 to a timing pulley 15 mounted on the rotatable shaftof a top ring motor 14 which is fixedly mounted on the top ring head 9.Therefore, when the top ring motor 14 is energized, the rotatablecylinder 11 and the top ring shaft 8 are integrally rotated through thetiming pulley 15, the timing belt 13 and the timing pulley 12. Thus thetop ring 1 is rotated. The top ring head 9 is supported by a top ringhead shaft 16 which is vertically fixed on a frame (not shown).

The presser ring 3 is corotatably, but vertically movably, coupled tothe top ring 1 by a key 18. The presser ring 3 is rotatably supported bya bearing 19 which is mounted on a bearing holder 20. The bearing holder20 is connected by vertical shafts 21 to a plurality of (three in thisembodiment) circumferentially spaced presser ring air cylinders 22. Thepresser ring air cylinders 22 are secured to a lower surface of the topring head 9.

The top ring air cylinder 10 and the presser ring air cylinders 22 arepneumatically connected to the compressed air source 24 throughregulators R₄ and R₅, respectively. The regulator R₄ regulates an airpressure supplied from the compressed air source 24 to the top ring aircylinder 10 to adjust the pressing force which is applied by the topring 1 to press the semiconductor wafer 4 against the polishing cloth 6.The regulator R₅ also regulates the air pressure supplied from thecompressed air source 24 to the presser ring air cylinder 22 to adjustthe pressing force which is applied by the presser ring 3 to press thepolishing cloth 6. The regulators R₄ and R₅ are controlled by acontroller (not shown in FIG. 5).

An abrasive liquid supply nozzle 25 is positioned above the turntable 5for supplying an abrasive liquid Q onto the polishing cloth 6 on theturntable 5.

As shown in FIG. 6, the top ring 1 has an outer circumferential annularflange 1s extending downwardly toward the turntable 5. The lower surfaceof the top ring 1 and the annular flange is jointly define a recess 1afor accommodating the semiconductor wafer 4 therein.

The polishing apparatus shown in FIGS. 5, 6 and 7 operates as follows:The semiconductor wafer 4 to be polished is placed in the recess 1a andheld against the elastic pad 2, and the top ring air cylinder 10 isactuated to lower the top ring 1 toward the turntable 5 until thesemiconductor wafer 4 is pressed against the polishing cloth 6 on theupper surface of the rotating turntable 5. The top ring 1 and thepresser ring 3 are rotated by the top ring motor 14 through the top ringshaft 8. Since the abrasive liquid Q is supplied onto the polishingcloth 6 by the abrasive liquid supply nozzle 25, the abrasive liquid Qis retained on the polishing cloth 6. Therefore, the lower surface ofthe semiconductor wafer 4 is polished with the abrasive liquid Q whichis present between the lower surface of the semiconductor wafer 4 andthe polishing cloth 6,

During polishing, compressed air is supplied from the compressed airsource 24 to the first, second and third chambers C₁, C₂ and C₃selectively, and the supplied compressed air is ejected from the lowersurface of the elastic pad 2 through the openings 1o and 2o, and issupplied between the holding surface of the top ring 1 and the uppersurface of the semiconductor wafer 4. At this time, at least one of thechambers C₁, C₂ and C₃ to which compressed air is supplied is selected,and at least one of the annular areas A₁, A₂ and A₃ from whichcompressed air is ejected is selected. For example, compressed air issupplied only to the first chamber C₁, and is not supplied to the secondand third chambers C, and C₃, whereby the semiconductor wafer 4 ispressed against the polishing cloth 6 by the compressed air in such astate that the polishing pressure applied to the central portion of thesemiconductor wafer 4 is larger than the polishing pressure applied toouter circumferential portion of the semiconductor wafer 4. Thus, if theamount of a material removed from the outer circumferential portion ofthe semiconductor wafer 4 is larger than the amount of a materialremoved from the central portion of the semiconductor wafer 4,insufficient polishing action at the central portion of thesemiconductor wafer can be corrected by utilizing the pressing action ofthe pressurized fluid.

On the other hand, if the amount of a material removed from the centralportion of the semiconductor wafer 4 is larger than the amount of amaterial removed from the outer circumferential portion of thesemiconductor wafer 4, the compressed air is supplied only to the thirdchamber C₃, and is not supplied to the first and second chambers C₁ andC₂, whereby the polishing pressure applied to the outer circumferentialportion of the semiconductor wafer 4 is larger than the polishingpressure applied to the central portion of the semiconductor wafer 4.Thus, insufficient polishing action at the outer circumferential portionof the semiconductor wafer can be corrected, and the entire surface ofthe semiconductor wafer 4 can be uniformly polished.

The pressures of compressed air supplied to the first chamber C₁, thesecond chamber C₂ and the third chamber C₃ are changed respectively,that is, compressed air having a pressure of p₁ gf/cm² is supplied tothe first chamber C₁, compressed air having a pressure of P₂ gf/cm² issupplied to the second chamber C₂, and compressed air having a pressureof p₃ gf/cm² is supplied. In this manner, the compressed air which issupplied between the holding surface of the top ring 1 and the uppersurface of the semiconductor wafer 4 has pressure gradient so as to behigher or lower progressively from the central area to the outercircumferential area of the semiconductor wafer 4. That is, the pressingforce for pressing the semiconductor wafer 4 against the polishing cloth6 has gradient from the central area to the outer circumferential areaof the semiconductor wafer 4. Thus, irregularities of the polishingaction can be sufficiently corrected and the localized area of thesemiconductor wafer 4 is prevented from being polished excessively orinsufficiently.

Further, in the present invention, depending on the pressing forceapplied by the top ring 1 actuated by the top ring air cylinder 10, thepressing force applied to the polishing cloth 6 by the presser ring 3actuated by the presser ring air cylinders 22 is adjusted while thesemiconductor wafer 4 is being polished. During the polishing process,the pressing force F₁ (see FIG. 1) which is applied by the top ring 1 topress the semiconductor wafer 4 against the polishing cloth 6 can beadjusted by the regulator R₁, and the pressing force F₂ which is appliedby the presser ring 3 to press the polishing cloth 6 can be adjusted bythe regulator R₂. Therefore, during the polishing process, the pressingforce F₂ applied by the presser ring 3 to press the polishing cloth 6can be changed depending on the pressing force F₁ applied by the topring 1 to press the semiconductor wafer 4 against the polishing cloth 6.By adjusting the pressing force F₂ with respect to the pressing forceF₁, the distribution of polishing pressures is made continuous anduniform from the center of the semiconductor wafer 4 to its peripheraledge and further to the outer circumferential edge of the presser ring 3disposed around the semiconductor wafer 4. Consequently, the peripheralportion of the semiconductor wafer 4 is prevented from being polishedexcessively or insufficiently. The semiconductor wafer 4 can thus bepolished to a high quality and with a high yield.

If a larger or smaller thickness of material is to be removed from theperipheral portion of the semiconductor wafer 4 than from the innerregion of the semiconductor wafer 4, then the pressing force F₂ appliedby the presser ring 3 is selected to be of a suitable value based on thepressing force F₁ applied by the top ring 1 to intentionally increase orreduce the amount of a material removed from the peripheral portion ofthe semiconductor wafer 4.

By controlling compressed air supplied to the first, second and thirdchambers C₁, C₂ and C₃, the semiconductor wafer 4 is polished by acombination of a pressing action caused by the compressed air and apressing action caused by the presser ring 3. Thus, insufficientpolishing action in the localized area (for example, the central area orthe outer circumferential area) of the semiconductor wafer can becorrected. Further, the amount of the material removed from thelocalized areas (for example, the central area or the outercircumferential area) can be intentionally increased or decreased. Inthis case, in the case where the polishing pressure at the centralportion of the semiconductor wafer 4 is made larger than the polishingpressure at the outer circumferential portion of the semiconductor wafer4, the pressing force F₂ of the presser ring 3 is made larger than thepressing force F₁ of the top ring 1. Conversely, in the case where thepolishing pressure at the outer circumferential portion of thesemiconductor wafer 4 is made larger than the polishing pressure at thecentral portion of the semiconductor wafer 4, the pressing force F₂ ofthe presser ring 3 is made smaller than the pressing force F₁ of the topring 1.

In this embodiment, since the semiconductor wafer 4 is accommodated inthe recess 1a of the top ring 1 and protected by the annular flange 1s,the outer circumferential surface of the semiconductor wafer 4 at itsperipheral edge is not rubbed by the presser ring 3 when the presserring 3 is vertically moved with respect to the top ring 1. Therefore,the presser ring 3 as it is vertically moved with respect to the topring 1 does not adversely affect the polishing performance of thepolishing apparatus during the polishing process.

FIG. 8 shows a polishing apparatus according to a second embodiment ofthe present invention. As shown in FIG. 8, a top ring 51 comprises amain body 52 and a ring member 54 detachably fixed by bolts 53 to alower outer circumferential surface of the main body 52. The top ring 51has a recess 51a for accommodating the semiconductor wafer 4. The recess51a is defined by a lower surface of the main body 52 and an innercircumferential surface of the ring member 54. The semiconductor wafer 4accommodated in the recess 51a has an upper surface held by the lowersurface of the main body 52 and an outer circumferential surface held bythe inner circumferential surface of the ring member 54. The presserring 3 is vertically movably disposed around the top ring 51.

The main body 52 of the top ring 51 has therein a circular first chamberC₁ at a central position thereof, an annular second chamber C₂ disposedat a radially outer side of the first chamber C₁, and an annular thirdchamber C₃ disposed at a radially outer side of the first chamber C₂.The first chamber C₁, the second chamber C₂ and the third chamber C₃ areconnected to the compressed air source (not shown) to allow compressedair to be supplied thereto in the same manner as the embodiment in FIGS.5 through 7. The main body 52 of the top ring 51 has a plurality ofopenings 52o which are communicated with the first chamber C₁, thesecond chamber C₂ and the third chamber C₃, respectively. An elastic pad2 also has a plurality of openings 2o which are in registry with theopenings 52o. Thus compressed air can be applied to the upper surface ofthe semiconductor wafer 4.

While the workpiece to be polished according to the present inventionhas been illustrated as a semiconductor wafer, it may be a glassproduct, a liquid crystal panel, a ceramic product, etc. Further, aspressurized fluid, pressurized liquid may be used. The top ring and thepresser ring may be pressed by hydraulic cylinders rather than theillustrated air cylinders. The presser ring may be pressed by electricdevices such as piezoelectric or electromagnetic devices rather than theillustrated purely mechanical devices.

As described above, the present invention offers the followingadvantages:

The distribution of the pressing force of the workpiece is preventedfrom being nonuniform at the peripheral portion of the workpiece duringthe polishing process, and the polishing pressures can be uniformizedover the entire surface of the workpiece. Therefore, the peripheralportion of the semiconductor wafer is prevented from being polishedexcessively or insufficiently. The entire surface of workpiece can thusbe polished to a flat mirror finish. In the case where the presentinvention is applied to semiconductor manufacturing processes, thesemiconductor devices can be polished to a high quality. Since theperipheral portion of the semiconductor wafer can be used as products,yields of the semiconductor devices can be increased.

In the case where there are demands for she removal of a larger orsmaller thickness of material from the peripheral portion of thesemiconductor wafer than from the inner region of the semiconductorwafer depending on the type of the semiconductor wafer, the amount ofthe material removed from the peripheral portion of the semiconductorwafer can be intentionally increased or decreased. Further, the amountof the material removed from not only the peripheral portion of thesemiconductor wafer but also the localized area (for example, centralportion or outer circumferential portion) can be intentionally increasedor decreased.

Although certain preferred embodiments of the present invention havebeen shown and described in detail, it should be understood that variouschanges and modifications may be made therein without departing from thescope of the appended claims.

1. A polishing apparatus for polishing a surface of a workpiececomprising: a turntable having a polishing surface thereon; a top ringfor supporting the workpiece to be polished and pressing the workpieceagainst said polishing surface under a first pressing force, said topring having a holding surface for holding the workpiece; a pressurizedfluid source for supplying pressurized fluid; a plurality of openingsprovided in said holding surface of said top ring for ejecting saidpressurized fluid supplied from said pressurized fluid source, aplurality of areas each having said openings being defined in saidholding surface so that said pressurized fluid is selectively ejectablefrom said openings in said respective areas.
 2. An apparatus accordingto claim 1, wherein said plurality of areas comprises concentric annularareas.
 3. An apparatus according to claim 1, wherein said plurality ofareas are defined by communicating with a plurality of chambers,respectively formed in said top ring through said openings.
 4. Anapparatus according to claim 1, wherein said first pressing force and apressure of said pressurized fluid are variable independently of eachother.
 5. An apparatus according to claim 1, wherein a pressure of saidpressurized fluid is variable in each of said areas.
 6. An apparatusaccording to claim 1, further comprising: a presser ring verticallymovably disposed around said top ring; and a pressing device forpressing said presser ring against said polishing surface under a secondpressing force which is variable.
 7. An apparatus according to claim 1,wherein said top ring has a recess defined therein for accommodating theworkpiece therein.
 8. A method of polishing a workpiece, comprising thesteps of: holding a workpiece between a polishing surface of a turntableand a holding surface of a top ring disposed above said turntable;pressing the workpiece by said top ring against said polishing surfaceunder a first pressing force; and ejecting pressurized fluid fromopenings in a plurality of areas in said holding surface of said topring toward the workpiece held by said top ring, said pressurized fluidbeing selectively ejectable from said openings in said respective areas;and polishing the workpiece in such a state that a pressing forceapplied to the workpiece by said pressurized fluid is variable in acentral portion and an outer circumferential portion of the workpiece,respectively.
 9. A method according to claim 8, further comprising thestep of: pressing a presser ring vertically movably disposed around saidtop ring against said polishing surface around the workpiece under asecond pressing force which is determined based on said first pressingforce.
 10. A method according to claim 8, said second pressing force isdetermined on the basis of a pressure distribution on the workpiececaused by said pressurized fluid ejected from said openings in saidrespective areas.
 11. A top ring for supporting the workpiece to bepolished, for use in a polishing apparatus, comprising: a holdingsurface for holding the workpiece; and a plurality of openings, providedin said holding surface, from which pressurized fluid is ejected, aplurality of areas each having said openings being defined in saidholding surface so that said pressurized fluid is selectively ejectablefrom said openings in said respective areas.
 12. A method of polishing asurface of a workpiece, comprising: holding a workpiece by a top ring;and pressing the workpiece against a polishing surface of a turntable topolish a surface of the workpiece by applying independently adjustablepolishing pressures to substantially concentric circular areas of theworkpiece, respectively, said polishing pressures including a firstpolishing pressure applied to a central circular area of the workpiece,a second polishing pressure applied to a first annular area of theworkpiece located outside of the central circular area, and a thirdpolishing pressure applied to a second annular area of the workpiecelocated outside of the first annular area.
 13. A method according toclaim 12, wherein said pressure is produced by air pressure.
 14. Amethod according to claim 12, further comprising applying an adjustablepressure to a presser ring vertically movably disposed around said topring for pressing said polishing surface.
 15. A method according toclaim 14, wherein said pressure applied to said presser ring is producedby air pressure.
 16. A method according to claim 12, wherein thepressure applied to a central portion of the workpiece is larger thanthe pressure applied to an outer circumferential portion of theworkpiece.
 17. A method according to claim 12, wherein the pressureapplied to an outer circumferential portion of the workpiece is largerthan the pressure applied to a central portion of the workpiece.
 18. Amethod of polishing a surface of a workpiece, the method comprising:holding a workpiece with a top ring; applying a first polishing pressureto a central portion of the workpiece; applying a second polishingpressure to an outer circumferential portion of the workpiece, whereinthe first polishing pressure is different than the second polishingpressure, and the first and second polishing pressures are independentlyadjustable; and pressing a presser ring against a polishing surfaceunder a pressing force which is variable.
 19. The method as claimed inclaim 18, wherein the first and second polishing pressures are producedby air pressure.
 20. The method as claimed in claim 18, wherein thefirst and second polishing pressures are produced by pressurized fluid.21. The method as claimed in claim 18, wherein the polishing pressureapplied to the central portion of the workpiece is larger than thepolishing pressure applied to the outer circumferential portion of theworkpiece.
 22. The method as claimed in claim 18, wherein the polishingpressure applied to the outer circumferential portion of the workpieceis larger than the polishing pressure applied to the central portion ofthe workpiece.
 23. The method as claimed in claim 18, wherein thepolishing pressures can be independently changed at different locationsof the workpiece.
 24. The method as claimed in claim 23, wherein thenumber of different locations where the polishing pressures can beindependently changed includes at least three different locations.
 25. Amethod of polishing a surface of a workpiece comprising: holding aworkpiece by a top ring; pressing the workpiece by the top ring againsta polishing surface; supplying fluid pressures in a plurality ofchambers formed in said top ring to provide polishing pressure to acentral area and an outer circumferential area, wherein a radial widthalong a radius of said top ring of an outer one of said plurality ofchambers is narrower than a radius of a central one of said plurality ofchambers; wherein said fluid pressures in said respective chambers arecontrollable independently of each other on the basis of the pressuresdetected by pressure sensors.
 26. The method as claimed in claim 25,further comprising pressing a presser ring against the polishingsurface.
 27. The method as claimed in claim 26, wherein the pressing ofthe pressing ring is achieved by applying an adjustable pressure to thepresser ring.
 28. The method as claimed in claim 24, wherein thepressure applied to the presser ring is produced by air pressure.
 29. Aworkpiece carrier for holding a workpiece and pressing the workpieceagainst a polishing surface, said workpiece carrier comprising: a topring for supporting the workpiece to be polished; a pressing mechanismfor pressing the workpiece against the polishing surface, said pressingmechanism being configured to apply a first polishing pressure to acentral circular area of the workpiece and a second polishing pressureto an annular area of the workpiece that is outside of the centralcircular area, wherein the first polishing pressure and the secondpolishing pressure are controllable independently of each other; and apresser ring vertically movably disposed around said top ring, saidpresser ring being pressed against a polishing surface under a pressingforce which is variable.
 30. The workpiece carrier as claimed in claim29, wherein said pressing mechanism comprises at least two pressurizedchambers to which pressurized fluid is supplied, said at least twopressurized chambers comprising a central circular chamber and a firstannular chamber located outside of said central circular chamber, saidcentral circular chamber and said first annular chamber beingpositionable over the central circular area and the first annular areaof the workpiece, respectively, wherein said first polishing pressureand said second polishing pressure can be created by said pressurizedfluid supplied to said central circular chamber and said first annularchamber, respectively.
 31. The workpiece carrier as claimed in claim 29,wherein said first polishing pressure and said second polishing pressureare controllable independently of each other during polishing.
 32. Theworkpiece carrier as claimed in claim 29, wherein said first polishingpressure and said second polishing pressure can be substantiallyuniformly applied to each of the central circular area and the firstannular area of the workpiece.
 33. The workpiece carrier as claimed inclaim 30, wherein said first polishing pressure and said secondpolishing pressure are controllable by varying said pressurized fluidsupplied to said central circular chamber and said first annularchamber, respectively.
 34. The workpiece carrier as claimed in claim 30,wherein said pressurized fluid comprises pressurized air.
 35. Theworkpiece carrier as claimed in claim 30, further comprising a secondannular chamber located outside of said first annular chamber, whereinsaid second annular chamber can be positioned over a second annular arealocated outside of said first annular area of the workpiece so that athird polishing pressure, created by pressurized fluid supplied to saidsecond annular chamber, can be applied to the second annular area of theworkpiece.
 36. The workpiece carrier as claimed in claim 35, wherein thepressurized fluid supplied to said second annular chamber comprisespressurized air.
 37. A method for polishing a surface of a workpiece,the method comprising: holding a workpiece by a top ring; pressing theworkpiece against a polishing surface by applying polishing pressure tothe workpiece, said polishing pressure including a first polishingpressure applied to a central circular area of the workpiece and asecond polishing pressure applied to a first annular area of theworkpiece located outside of said central circular area, said firstpolishing pressure and said second polishing pressure being controllableindependently of each other; and pressing a presser ring against saidpolishing surface under a pressing force which is variable.
 38. Themethod as claimed in claim 37, further comprising applying a thirdpolishing pressure to a second annular area of the workpiece locatedoutside of the first annular area of the workpiece, the third polishingpressure being controllable independently of the first and secondpolishing pressures.
 39. A method for polishing a surface of aworkpiece, the method comprising: holding a workpiece by a top ring;pressing the workpiece against a polishing surface by applying polishingpressure to the workpiece, the polishing pressure including a firstpolishing pressure applied to a central circular area of the workpieceand a second polishing pressure applied to a first annular area of theworkpiece located outside of the central circular area of the workpiece,the first polishing pressure and the second polishing pressure beingcontrollable independently of each other; and pressing an area of thepolishing surface around the workpiece with a presser ring under apressing force which is variable.
 40. A method for polishing a surfaceof a workpiece, the method comprising: holding a workpiece by a topring; and pressing the workpiece against a polishing surface by applyingpolishing pressure to the workpiece, the polishing pressure including afirst polishing pressure applied to a central circular area of theworkpiece, a second polishing pressure applied to a first annular areaof the workpiece located outside of the central circular area, and athird polishing pressure applied to a second annular area of theworkpiece located outside of the first annular area; and pressing apresser ring against said polishing surface under a pressing force whichis variable.
 41. A polishing apparatus for polishing a surface of aworkpiece, comprising: a top ring for holding a workpiece, and aplurality of chambers formed in said top ring, fluid pressures beingsupplied in said respective chambers to provide polishing pressure to acentral area and an outer circumferential area, wherein a radial widthalong a radius of said top ring of an outer one of said plurality ofchambers is narrower than a radius of a central one of said plurality ofchambers.
 42. A polishing apparatus according to claim 41, wherein saidpolishing pressure comprises a fluid pressure.
 43. A polishing apparatusaccording to claim 41, wherein said polishing pressure comprises acompressed air pressure.
 44. A polishing apparatus according to claim41, wherein said central area and said outer circumferential area areformed by chambers formed in said top ring.
 45. A polishing apparatusaccording to claim 41, further comprising: a third area located betweensaid central area and said outer circumferential area.
 46. A polishingapparatus according to claim 41, further comprising: a presser ringdisposed around the workpiece and contacting a polishing surface forpolishing the workpiece.
 47. A polishing apparatus according to claim46, wherein said presser ring is pressed by pressurized fluid.
 48. Amethod for polishing a workpiece, comprising: holding a workpiece on asurface of a top ring; and applying a polishing pressure on a surface ofthe workpiece, wherein an area where said polishing pressure is appliedis divided into at least a central area and an outer circumferentialarea of the workpiece, and a radial width along a radius of said topring of the outer circumferential area is narrower than a radius of thecentral area.
 49. A method according to claim 48, wherein said polishingpressure comprises a fluid pressure.
 50. A method according to claim 48,wherein said polishing pressure comprises a compressed air pressure. 51.A method according to claim 48, wherein said central area and said outercircumferential area are formed by chambers formed in said top ring. 52.A method according to claim 48, wherein the area where said polishingpressure is applied is divided to said central area, said outercircumferential area and a third area located between said central areaand said outer circumferential area.
 53. A method according to claim 48,further comprising: pressing a presser ring disposed around theworkpiece against a polishing surface for polishing the workpiece.
 54. Amethod according to claim 53, wherein said presser ring is pressed bypressurized fluid.